Nouns such as 'bird', 'tree', 'boy', 'cow', which means one are called singular nouns. On the other hand nouns such as 'birds', 'trees', 'boys', 'cows', which means more than one are called plural nouns.
In the above examples, the nouns ball, apple, tree, bird, pencil and car stand for only one person, animal or thing. They are, therefore, called singular nouns. The nouns balls, apples, trees, birds, pencils and cars stand for more than one person, animal or thing. They are, therefore, called plural nouns.